I used a Lee soft 0.9 GND filter.
This picture is inspired in the work of my favourite landscape photographer: Atomic Zen. I studied in detail all his advices on each of the pictures he has been publishing in 500px. The best advice I could take from him is to forget about HDR technology when shooting seascape photography. It is amazing what can be done simply with the right processing over a RAW file.
Thanks to Atomic Zen for sharing with the 500px community all his hints.